
Class Rm^dH 



PRESENTED BY 



PROCEEDINGS 

OF THE 

WASHINGTON ACADEMY OF SCIENCES 

Vol. VIII, pp. 91-110. PLS. IV-V July 10, 1906 



EVIDENCE BEARING ON TOOTH-CUSP 
DEVELOPMENT 



James Williams Gidley 



WASHINGTON, D. C. 

Published by the Academy 

1906 



PROCKKDINGS 



OF THE 



^^3 



WASHINGTON ACADEMY OF SCIENCES 

VoT>. VIII, pp. 91-106 PLS. iv-v July 10, 1906 



EVIDENCE BEARING ON TOOTH-CUSP 
DEVELOPMENT/ 

By James Williams Gidley, 
Department of Geology, U. S. National Museum. 

In connection with the work of cataloguing the portion of the 
Marsh collection of Mesozoic mammals, ohtained under the au- 
spices of the U. S. Geological Survey and now deposited in the 
United States National Museum, I have made some discoveries 
of seeming importance in the form of evidence bearing on the 
question of tooth-cusp homologies in the mammalian molars. 
This evidence I wish briefly to present in the following pages, 
hoping it may throw some added light on the very important 
subject of tooth morphology. 

Before proceeding, I wish to express my indebtedness to Dr. 
George P. Merrill for making possible the arrangements for 
this detailed study of material and for his encouragement in the 
work ; to Prof. Charles Schuchert, of Yale University, for 
submitting to my hand the type material of the Marsh collection 
at New Haven ; and to Prof. Henry F. Osborn of the American 
Museum of Natural History, for his courtesy in placing the 
collection of Mesozoic mammals in that institution at my disposal. 
My thanks are also due Mr. G. S. Miller, Jr., for his valuable 
aid in selecting study material from the collection of modern 
mammals in the National Museum and for a clear translation 
of Herluf Winge's paper on tooth-cusp development. 

1 Based on a study of the Mesozoic Mammal Collection in the U. S. National 
Museum. 

Proc. Wash. Acad. Sci , July, 1906. 91 



92 GIDLEY 

Of the several theories thus far advanced for the evolution of 
the teeth, none has been entirely satisfactory, and there is still 
a wide disagreement among authorities, especially as regards 
the position of the primary cone or '* protocone " in the upper 
molars. As proposed by the late Prof. E. D. Cope and sup- 
ported by Prof. Henry F. Osborn, the primary cone is to be 
found invariably on the inner or lingual side of the trigonodont 
upper teeth, and is the homologue of the central cone in Tri- 
conodon^ in which the three main cusps are arranged in an 
antero-posterior line, the trigonodont molar having been derived 
from this form through the shifting of the two lateral cones to 
the outside. The central cone {protocone) remaining on the 
inner side, thus forms a triangle (trigon) with the apex pointing 
inward. In the meantime, according to this theory, the cusps 
of the lower molars are supposed to have moved in the opposite 
direction, leaving the central cusps {frotoconid) on the outside, 
forming an oppositely directed triangle (trigonid). Thus the 
primary cones of the upper and lower molars in shifting have 
completely reversed their positions in relation to each other, the 
primary cone of the upper molars not only moving to the inner 
side of the crown, but taking a position in the series inside the 
primary cone of the lower molars as well. 

This theory, so skillfully worked out by Osborn, has been 
widely accepted as satisfactorily explaining the problem of tooth- 
cusp evolution. But recent paleontological and embryological 
investigations have thrown a large amount of discredit on the 
whole theory. As stated by Wortman, Scott has shown most 
conclusively, from paleontological evidence, that in the upper 
molariform premolars the primary cone is on the outer side and 
the subsequently added cusps have a very different history from 
that proposed by the tritubercular theory for the true molars. 
The embryological researches of Woodward, Tacker, and others 
have not only confirmed Scott's theory for the premolars, but 
show also that in all groups of mammals investigated the antero- 
exlernal cusp or paracone is first to appear in the permanent 
upper molars and milk molars, as it does in the premolars, and 
the order of appearance of the other principal cusps is practi- 
cally the same as proposed by Scott for the premolars. 



EVIDENCE BEARING ON TOOTH-CUSP DEVELOPMENT 93 

Woodward' found that in Centetes and E/'icidus the main in- 
ternal cusp, usually termed the protocone, was nrst to develop, 
but he believed this cusp to be the paracone, the whole tooth 
representing only the antero-external triangle of such a form 
as Talpa^ the protocone and metacone not having been de- 
veloped. This, as stated by Woodward, is a moditication of 
Mivart's view published in i86S,- in which he states his belief 
that in Centetes, Chrysocloris'" and like forms, the main 
portion of the crown represents the union of the two external 
prisms of Talpa and like forms. According to Mivart, the 
main internal cusp of Centetes^ Ej'icuIus^ Chrysocloris, etc., 
was derived bv the fusion of the paracone and metacone, while 
the protocone and hypocone are wanting or rapidly diminishing 
in size and importance. According to both Woodward and 
Mivart, therefore, in these forms, which have been considered 
typical trituberculates, the outer cusps are developments of the 
cingulum, while the main internal cusp has been wrongly termed 
the protocone and is in realit}' the paracone, according to 
Woodward, or combined paracone and metacone, according to 
Mivart, while the inner cusp (protocone) is greatly diminished in 
size or has entirely disappeared. These two authorities, there- 
fore, are agreed on the two points of principal importance regard- 
ing Centetes and Ericulus, viz : uj the location of the paracone in 
the main internal cusp and (2) the ultimate loss of the protocone. 
I strongly concur in these views, for in a series of upper molars, 
including Potamogale^ Solenodon, Centetes, Ericulus^ Hemi- 
centetes and Chrysocloris (see figs. 1-6, pi. IV), the stages sug- 
gesting the gradual diminishing and final disappearance of the 
protocone are verv* complete, amounting almost to demonstration, 
and there can be little doubt that the molars of the Centetes and 
Chrysocloris type have been derived from forms similar to that 
of Potamogale^ involving the loss of the protocone. In conse- 
quence of this the paracone, or combined paracone and meta- 
cone, comes to be the principal inner cusp. In Potaniogale the 

^ Proc. Zool. Soc. London 1S96, 5SS-5S9. 

^Journ. Anatomy and Phvsiol.. Vol. 11. 139, 1S6S. 

^The form figured by Mivart has since been removed to a distinct genus, 
Bematiscus Cope. Am. Xat.. XX\"I, 1S92. 127. The typical Chrysocloris upper 
molar has no trace of a r)rcrocor.e. 



94 GIDLEY 

protocone is quite prominent and still typical in form, while in 
Solenodo7i it is much reduced and is beginning to divide trans- 
versely, or more probably is beginning to separate from a like- 
wise reducing hypocone. This is in favor of the view held 
by Mivart that the simple inner cusp in Potamogale and like 
forms is in reality the fused protocone and hypocone. The 
reduction is carried still further in Centetes, in which two 
inner cingulum-like cusps appear, one on each side of the 
enlarged paracone. In Chrysocloris and He7nicentetes the 
inner cusp (protocone and hypocone) has entirely disappeared. 
Regarding Mivart's '' fusion theory," I am inclined to believe 
that Woodward has not given due weight to the evidence cited 
by Mivart and that there is considerable support for this theory 
to be found in the modern bats and insectivores. Mivart con- 
sidered the Potamogale molar as an intermediate form between 
molars of the Talfa type, having two external triangular prisms, 
and those of Centetes and Ericulus^ having only one such 
prism. He pointed out that in Potamogale there is **avery 
interesting approximation of the triangular prisms," in which 
the paracone and metacone, although still remaining distinct, 
are in very close juxtaposition. This view is strongly supported 
by a series of bat molars to which Mr. G. S. Miller has kindly 
called my attention. In this series, which includes Vespertilw, 
Scotofhilus and Harf>iocefhalus^ are suggested the successive 
steps from Talfa to Potamogale in the insectivore group. 
Vesfertilio represents the normal or more generalized form, 
in which the protocone is large, the paracone and metacone 
are widely separated, and the external styles are nearly equal 
in size. The mesostyle is much reduced in Scotophtlus and is 
drawn inward, the paracone and metacone are more closely 
appressed and the protocone is somewhat shortened. In Har- 
fiocefhaliis ^ the mesostyle has disappeared, the parastyle and 

iThe skull of Harpiocephaliis from which this description was taken was 
obtained by Mr. G. S. Miller through the kindness of Oldfield Thomas, of the 
British Museum. 

Unforunatelj it came too late to be photographed and figured uniformly 
with the series. Its place is taken on Plate III, by an outline drawing from a 
figure for Wilhelm Peters' Fledermause des Berlines Museums fiir Naturkunde 
(a projected monograph of the bats). 



EVIDENCE BEARING ON TOOTH-CUSP DEVELOPMENT 95 

metastyle have drawn closer together and compose the entire 
outer portion of the crown, while the paracone and metacone 
are closely approximated, forming the greater part of the inner 
portion of the crown, the protocone being very much reduced. 
Thus in Harfioce^halus a stage is reached nearly analogous 
to that of Potainogale, the principal difference being that the 
metacone is the dominant cusp instead of the paracone, as in 
the latter genus. ^ 

From these comparisons it seems reasonably clear that such 
forms as Centetes, Ericulus and Chrysochloris have attained a 
secondary or pseudo-tritubercular form by passing through some 
such stages of evolution as are suggested by the two series here 
selected. Other examples of a fusing paracone and metacone 
and reducing protocone may be found in the molars of some of 
the creodonts and carnivorous marsupials and in the sectorials 
of many of the carnivores. 

From the foregoing it now seems to be demonstrated beyond 
question that the main inner cone of Centetes and Erictihis is 
not the protocone as observed in normal groups, but, if not 
entirely made up of the primary cusp (paracone), it at least in- 
volves that element and Woodward's contention that the evi- 
dence of embryology is in entire harmony for the molars and 
pren:iolars is not controverted by these seeming exceptions as 
supposed by Osborn. 

Wortman of late has strongly opposed what he terms the 
*'cusp migration theory," and has brought considerable evi- 
dence to show that, in the creodonts and carnivores, at least, 
the cusps of the upper molars in general are homologous to 
those of the molariform premolars and have had substantially 
the same history in their development. 

Against this combined evidence Osborn ^ has recently re- 
affirmed the tritubercular theory, *« as originally proposed," 
resting the whole question on the point of evidence as to 
** whether the main reptilian cone, or protocone, of the ances- 

1 In the Laramie mammals I find that the metacone equals or is larger than 
the paracone in those forms in which the postero-external heel is well developed 
in the upper molars. 

2 Amer. Journ. Science (4), Vol. 17, 1904, 321-323. 



96 GIDLEY 

tors of the mammals was found upon the antero-internal side or 
on the antero-external side of the upper molars." This evidence, 
according to Osborn, is in favor of the tritubercular hypothesis, 
and conclusive evidence of the theory is furnished in the Jurassic 
mammal molars. However, a study of all the mesozoic mam- 
mal material available has led the present writer to exactly 
opposite conclusions. 

Unfortunately, Osborn's observations were confined to a very 
limited amount of material, and from a careful examination of 
the teeth of Triconodon and Dryolestes^ two forms especially 
studied by him, it seems that his conclusions were based on 
evident, though perfectly excusable, errors of observation, due 
doubtless to the minuteness of the teeth and their dark color, 
which make it difficult in many cases to distinguish, between a 
fracture and the natural surface of the tooth. Thus, according 
to Osborn,^ the upper molars of Dryolestes are *' broadly trans- 
verse or triangular and upon the mternal ^ido. of each is a large, 
conical, pointed cusp,_^r, supported by a large stout fang, . . . 
The external portion of the crown is depressed, and bears one 
large antero-external cusp 1 pa and one smaller postero-external 
cusp ? 7ne which is either partially worn away or less pronounced 
in development." But there are two important cusps not noted by 
Osborn, one an external cusp placed anterior to the main external 
cusp, the other a small but well-defined intermediate cusp appear- 
ing on the posterior transverse ridge. Thus there are five distinct 
cusps instead of three, as stated by Osborn, and these do not form 
a trigon in the sense that this term has been used, for the main 
external cusp is in the middle pf the base of the triangle instead 
of forming one of its angles. 

In the upper molars of Triconodon the three principal cusps are 
arranged in a direct line, and are nearly equal in size and form, 
and the two lateral cones are each supplemented by a small but 
well-defined internal basal heel-like cusp and an external basal 
cingulum. The main cusps are flattened externally into a con- 
tinuous wall in one species (see PI. V, fig. i), while they are 

^The specimens studied by the present writer and referred to these genera 
are from the Atlantasam'us beds of Wyoming. These beds are usually referred 
to the upper Jurassic, although they may be lower Cretaceous. 

^Amer. Journ. Science (4), Vol. 17, 1904, 322. 



EVIDENCE BEARING ON TOOTH-CUSP DEVELOPMENT 97 

much rounded and deeply divided on the inner or lingual side. 
Thus, there is not the slightest suggestion of a tendency toward 
an outward movement of the lateral pair of cusps, while it is 
easily conceivable that the continued development of the two 
inner heel cusps and outer cingula would early result in a gen- 
eral form of tooth very different in pattern from the tritubercular 
type which might form the basis for such molars as those of the 
diprotodont marsupials and many of the rodents or even of the 
manatee and mastodon. I do not wish to be understood here 
as implying any relationship between these very diverse forms, 
but as especially emphasizing the fact that in Triconodon is sug- 
gested an easy and not improbable way in which some complex 
molars may have been derived without having passed through 
the typical tritubercular stage. 

Thus, it is shown by this restudy of the two forms, which 
according to Osborn represent successive stages in the evolution 
of the mammalian molar, that the gap between them, which 
was already great, even according to Osborn's interpretation, is 
very greatly increased especially from the tritubercular theory 
standpoint. Moreover there is no evidence, in the way of in- 
termediate forms, indicating \}[\'dXDryolestes ever passed through 
a stage strictly analogous to that of Triconodon or that the 
main internal cusp is in any way homologous to the central 
cone 'in the Triconodon molars. Furthermore, a critical com- 
parison of these two forms shows that such an hypothesis is beset 
by many difficulties. The following are the principal ones : 
(i) The molars of Triconodon are larger and fewer in number 
than in Dryolestes indicating a generally higher specialization. 
(2) The lateral cones in Triconodon are already comparatively 
much specialized, being suplemented by growths of the cingu- 
lum externally and heel cusps internally and thus do not es- 
pecially resemble, either in form or proportions, any two of the 
external cusps in Dryolestes. (3) The external portion of the 
upper molar \n Dryolestes (see PI. V, figs. 2 and 3) is composed 
of three simple connate cusps supported by two fangs, their 
general appearance suggesting an arrangement homologous to 
the three cusps and two fangs of Triconodon; while (4) the 
internal portion of the tooth is a high antero-posteriorly com- 



98 



GIDLEY 



pressed V-shaped cusp supported by a single fang, centrally 
placed, and exposed on its inner side for the greater part of its 
length, the maxillary bone apparently not yet having formed 
a completed socket, or alveolus, for its reception. Thus the 
whole construction of the inner cusp, which is highly sugges- 
tive of a heel development, differs materially from the central 
cone of Triconodon. 



o o o 



B 



ch3h> oOo KD^ 



c 



o-=Q=o 0=0=0 



D 




F 



V 








J 



Fig. II. Phyletic History of the Cusps of the Ungulate Molars. A, Reptilian 
Stage, Haplodont, Permian. B, Protodont Stage {Drotnothertum) , Triassic. 
C, Triconodont Stage {Amphilestes). D, Tritubercular Stage {Spalacothe- 
rium). E, Tritubercular-tuberculo Sectorial, Lower Jurassic. F, The same, in 
Upper Jurassic. G, The same, in Upper Cretaceous. //, The same, Puerco, 
Lower Eocene. /, Sexitubercular-sexitubercular, Puerco. /' Sexitubercular- 
quadritubercular, Wahsatch. (After Osborn.) 

Considering the outer portion of the Dryolestes molar as 
homologous to the three cones and two fangs of Triconodon^ 



EVIDENCE BEARING ON TOOTH-CUSP DEVELOPMENT 99 



^ ^ 00 o 



^ 000 



' ctf^fc^"^ 5c,,^;;;:^3=o 



E 4 




^J^ 



Q^K^c^K) 



C 5 






^^ 




Fig. 12. Suggested Phyletic History of Tivo Ty^es of Cojuplex Molars. [As 
in Osborn's diagram, the solid black dots represent the cusps of the upper 
molars, the circles, those of the lower molars.] i to 6, Phjletic history of the 
" Tritubercular " type; a to d, Phyletic history of the " Triconodont " type; 
e,f From the brachyodont Triconodont stage to the bilobed hypsodont type of 
molar. 

A, B, C, E and G compare with A, B, C, E and G in Osborn's diagram, fig. 
II ; 4, Dryolestes type, Atlantosaurus beds (.? Upper Jurassic) ; 5 and 6, Proto- 
lamhda or Pediojnys type, Laramie beds (Upper Cretaceous) ; d, Triconodon 
type, Atlantosaurus beds ( 1 Upper Jurassic) ; f Palceolagiis type, White River 
beds (Oligocene). 



lOO GIDLEY 

the derivation of this type of tooth is much simplified, it being 
not so far removed from the primitive reptilian condition, and 
though diverging on different lines, is no more specialized, as 
a whole, than the Triconodon type of tooth, the differentiation 
being carried on more rapidly in the latter in the special de- 
velopment of the anterior and posterior lateral cones and their 
accessory cusps, while in Dryolestes the specialization has 
apparently been centralized in the development of the high, 
narrow, heel-like cusp and its supporting fang on the inner 
side of the molar. 

This view is strongly supported by the evidence obtained from 
still another characteristic Atlantosaurus-beds type of molar 
represented by Dicrocy^iodon. In this form, PL V, fig. 4, the 
same primitive arrangement of three cusps and two fangs is 
preserved in the outer portion of the tooth, while on the internal 
side a large secondary cusp has been developed differing widely 
in character from that of Dryolestes, This cusp is a laterally 
compressed cone supported by two rudimentary fangs and is 
joined to the outer portion of the tooth by a high, wedge-shaped 
ridge. The base of the inner cone is greatly expanded antero- 
posteriorly, curving gently outward toward the external portion 
of the tooth. Thus the crown, as a whole, is greatly constricted 
medially with the inner and outer portions superficially resem- 
bling each other. 

From these observations two important conclusions may be 
drawn : First, that, leaving out of consideration the multitu- 
berculates, there are among the mammals of the Atlantosaurus 
beds at least three distinct forms of upper molars representing 
three primitive types of about equal specialization apparently 
leading off in entirely independent lines. Probably only one of 
these, Dryolestes^ represents an ancestral type from which the 
Upper Cretaceous and later forms possessing trigonodont molars 
may have been derived. Second, that the evidence derived 
from the Atlantosaurus beds mammals entirely supports the 
evidence of embryology and agrees in general with the ** pre- 
molar analogy " theory. Thus, the evidence from all sources 
points overwhelmingly to the conclusion that the primary cone 
is to be found on the outer side in the upper molars of primi- 



EVIDENCE BEARING ON TOOTH-CUSP DEVELOPMENT lOI 

tive trituberculate forms and in all forms derived from a tritu- 
bercular type of tooth as well, except where the main inner cone 
(protocone) has been reduced secondarily. The opposite view 
held by the tritubercular theory now apparently stands on very 
insufficient evidence, and the proposition that the protocone, of 
Osborn, represents the primary cusp is entirely without support. 
The lower molars of the Atlantosaurus beds mammals fur- 
nish abundant additional evidence along the line of conclusions 
regarding the shifting of three cusps from a straight line to 
form the primitive triangle. In such forms as Dryolestes and 
Paiirodon we have trituberculate molars in the primitive or 
forming stage, and, what is most significant, the cusps resemble 
very closely, both in position and relative proportions, those of 
the premolars of later types in their early stages of transition 
to the molariform pattern. In the lower molars of Pau7'odon 
the crown consists of a high, pointed cusp (protoconid), centrally 
placed, a low posterior heel, a small antero-internal cusp (para- 
conid), and a very small median internal cusp (metaconid). 
The last two form the base of the trigonid. In Dryolestes both 
the trigonid and the pimitive heel are somewhat more advanced 
in development. In still other forms, such as Manacodon and 
Tinodon^ the two internal cusps are relatively large and the 
trigonid is fully developed, while the heel, or talonid, is very 
small or entirely wanting. In all the paraconid and metaconid 
are entirely on the internal side of the crown, and in these and 
all the material examined there is not the slightest evidence 
of any shifting of the cusps, but they seem to have arisen in the 
positions they now occupy.^ In Paiirodon the heel is apparently 
as much or more developed than either of the internal cusps 
and seems to have made its appearance even in advance of the 
metaconid. Also the metaconid is still very rudimentary and is 
just budding off near the base of the protoconid, but little pos- 
terior to its apex and midway of the entire length of the crown, 
while the place of origin assigned to it by the tritubercular 
hypothesis is already occupied by the comparatively large heel. 

^ This is in accord with the general conclusions on tooth cusp development 
reached by Herluf Winge as early as 1882. Widinsk Meddelelsor fn den natur- 
hist. Florening e Kjobenhavn, 1S82, p. 18. 



102 GIDLEY 

From these observations it seems apparent that the trigonid 
of the lower molars is not the reverse of the trigon of the upper 
molars, as held by advocates of the tritubercular theory, and 
the homologues of the elements of the upper and lower molars, 
as proposed by this theory, are far from being apparent. (This 
also accords with the conclusions of Winge.) 

The lower molars of Triconodon differ from any of the forms 
just described. They are composed of three nearly equal 
cone-like cusps arranged like thovse in the upper molars of this 
genus in an antero-posterior line. There is no cusp corres- 
ponding with the metaconid in Dryolestes, There is a continu- 
ous basal cingulum on the inner face of the crown, and the 
posterior cusp is in no way homologous, except in position, to 
the heel in the lower molars of Paurodon and Dryolestes, 

The mammals from the upper Cretaceous Laramie beds show 
a great advance in development. The molars of the tritubercu- 
late forms of this horizon have passed into a second well-defined 
stage of specialization which, though varying greatly in detail 
in the various types, conforms in general to a distinctive pattern 
which may readily have been derived from some Atlantosaurus- 
beds form, such as Dryolestes. An upper molar of Pediomys 
Marsh, a typical example of the Laramie tritubercular molar, 
compared with the corresponding tooth of Dryolestes^ presents 
the following differences and indicates the principal lines of 
progression : 

(i) The main internal cusp {frotocone) is much broadened 
antero-posteriorly ; (2) a second small V-shaped intermediate 
cusp (^frotoconule) has been added ; (3) the postero-external 
cusp (?netacone) has greatly increased, nearly equaling, both in 
size and importance, the median external, or primary, cone 
[paracone), while the antero-external cusp (^farastyle) has re- 
mained small and undeveloped. A correspondingly pro- 
gressive development marks the trigonid and heel of the 
lower molars. 

Thus, the *' trigonodont" tooth, or a type of molar with three 
principal cusps of almost equal importance, arranged in the 
form of a triangle, makes its first appearance in the Laramie 
This pattern of tooth Cope early recognized as a general primi- 



EVIDENCE BEARING ON TOOTH-CUSP DEVELOPMENT IO3 

tive type, and on its representatives in the lower Eocene he 
founded the tritubercular theory. That this type is primitive 
and many, at least, of the later forms have been derived from 
it, have been too conclusively demonstrated by Cope, Osborn, 
Scott and others to be seriously questioned ; but this early 
trigonodont form, as is now evident, was derived in a totally 
different way from that assumed by the tritubercular hy- 
pothesis. 

An especially interesting feature in these Laramie forms is 
the oft-repeated appearance in the upper molars of a back- 
wardly extended outer heel-like cusp connected by an elevated 
ridge with the postero-external cusp. This portion of the tooth 
is thus converted into a more or less perfect sectorial, or cutting, 
blade, against which the anterior blade of the trigonid shears, 
while the greatly broadened heel or talonid of the lower molar, 
extending backward under the antero-posteriorly expanded 
protocone of the upper molar, forms a successful crushing 
apparatus. Thus, so early as the Cretaceous the prevailing 
molar types were about equally equipped for use as cutting or 
crushing mechanisms. The creodonts and carnivorous marsu- 
pials seem to have early taken advantage of the sectorial blade 
to the neglect of the crushing heel which gradully diminished 
in relative size and importance, while in many other forms, 
using -the crushing portion of the tooth most, the sectorial blade 
was early lost. 

Another special character marking the advance of the upper 
Cretaceous mammal molars is the first indication in a few forms 
of the postero-internal cusp {Jiyfocdne)^ which forms the fourth 
main cusp in the later quadra-tubercular type of molars. This 
cusp has apparently been derived, according to the evidence of 
these Laramie types, from independent sources in different 
groups of mammals. In a form which Marsh has referred to 
Telacodon a strong cone-shaped cusp has developed on the 
postero-internal cingulum of the tooth indicating the deriva- 
tion of the hypocone from that source. Another form, appar- 
ently representing an undescribed genus (PL V, fig. 7) is 
evidently developing a hypocone from the primitive posterior 
intermediate cusp. Still another form, represented by Proto- 



I04 GIDLEY 

lambda Osborn, seems to indicate a third source from which the 
hypocone may have developed. In Protolambda the internal 
heel (protocone) is broadly expanded and flattened posteriorly 
vs^ithout a cingulum, yet the peculiar shelf-like form of this por- 
tion of the tooth suggests the origin of a hypocone budding off 
from the protocone independently of either the cingulum or pos- 
terior intermediate cusp. 

From such a form as that presented in PL V, fig. 7, it is but 
a short step to the typical selenodont artiodactyl type of molar 
through the progressive development of the V-shaped posterior 
intermediate cusp. The addition of a second posterior cusp 
budding off from the enlarged postero-intermediate cusp would 
readily convert the tooth into a perissodactyl type of molar. 
Thus is suggested a fourth possible source of origin for the 
hypocone. This does not necessarily imply an actual relation- 
ship of this particular form to the ungulates, but indicates a 
type closely resembling them which differs widely from the 
primitive carnivores and insectivores, in which the hypocone, 
when present, was undoubtedly derived from the cingulum. 
These observations suggest especially that apparently homol- 
ogous elements in the teeth of the more highly complex forms 
may often arise from different sources. 

The correlation and homologies of the cusps of the lower 
molars in comparison with those of the upper series have, for 
the most part, been left out of this discussion. One observa- 
tion, in this connection, however, of seeming great importance 
and significance should be noted here. 

In examining a large number of examples of both living and 
extinct forms, I have found the following associations between 
the heel of the lower molars and the protocone of the upper 
teeth to hold constantly true, viz: A functional, broad, crush- 
ing protocone is invariably associated with a well-developed 
crushing heel in the opposing lower molar. A reduced or vesti- 
gial protocone is invariably associated with a correspondingly 
reduced or vestigial heel in the opposing lower molar. Since 
the heel of the lower molars is admittedly of secondary origin, 
this feature alone would seem to argue strongly for a like sec- 
ondary origin for the protocone in the upper molars. 



EVIDENCE BEARING ON TOOTH- CUSP DEVELOPMENT IO5 
SUMMARY AND CONCLUSIONS. 

Summing up the evidence derived from this preliminary 
study, the follov^ing conclusions are suggested : 

1. That the evidence obtained from the Mesozoic mammal 
teeth furnishes no support ,to the tritubercular theory in so far 
as it involves the position of the protocone and the derivation of 
the trigonodont tooth from the triconodont stage through the 
shifting of the lateral cones outward in the upper molars and 
inward in the lower molars. 

2. That it supports entirely the embryological evidence that 
the primary cone is the main antero-external cusp, or ;paracone, 
having retained its position on the outside in most upper molars 
(see exceptions above, p. 95). 

3. That it agrees in the main with Huxley's **premolar- 
analogy" theory, as supported by Scott. 

4. That the molars of the Multituberculates, Trtconodon, 
Dryolestes and Dtcrocynodon, were apparently derived inde- 
pendently from the simple reptilian cone ; hence the supposi- 
tion follows that the trituberculate type represents but one of 
several ways in which the complex molars of different groups 
may have been derived.^ 

5. That in the forms derived from the trituberculate type of 
molar the order of succession of the cusps is not the same in all 
groups, and apparently homologous elements are sometimes de- 
veloped from different sources. Hence it follows that no theory 
involving an absolute iinifonnity of succession in the development 
of complex molars will hold true for all groups of mammals . 

In the foregoing pages I have restricted the use of Osborn's 
tooth-cusp nomenclature for the reason that, in this particular 
discussion, there are some cases in which it is not strictly appli- 
cable and might lead to confusion. 

On similar grounds Dr. Wortman^ has expressed the opinion 
that all attempts to establish a tooth-cusp nomenclature founded 
on supposed homologies are *' foredoomed to failure" and 
should be entirely abandoned as *' useless and confusing." I 

^ Somewhat similar conclusions have been reached from different reasoning 
bj E. S. Goodrich, M. Tims and others. 

^Amer. Journ. Science (4), Vol. 16, 1903, 265-368. 



I06 GIDLEY 

agree with the general sentiment expressed {of. ctt,, p. 366) 
that, owing to the adoption of different plans in different groups 
of mammals for increasing the complexity of their molars, no 
terminology founded on the basis of cusp homologies can be 
made strictly applicable to all the mammalia. I do not, how- 
ever, consider this sufficient ground for abandoning absolutely 
so convenient a system of nomenclature as that proposed by 
Osborn. Granting that many of the terms proposed are founded 
on mistaken homologies, it does not necessarily follow that they 
need be in the least confusing, as suggested by Wortman. For 
in any system used, in order to make that system of greatest 
convenience and highest utility, the names once adopted should 
be permanent and not subject to transfer or substitution on any 
ground of changed conceptions of homologies or history, for 
the same reason that generic and specific names are retained 
regardless of the fact that they may have been given to denote 
some supposed affinity or characteristic which may later have 
proved entirely erroneous. 

Viewed from the nomenclature standpoint, therefore, the 
convenient names proposed by Osborn have come to assume an 
individuality which conveys a far more definite meaning than 
any purely descriptive terms, be they of relative position or 
supposed homologies. Moreover, they have the valuable ad- 
vantages of clearness and brevity in description. On these 
grounds, in the opinion of the present writer, and for the added 
reason that great confusion would inevitably result from any 
change in a terminology that has found its way into so many 
publications, Osborn's nomenclature should be retained as orig- 
inally proposed. Thus the term **protocone" always means 
the main antero-internal cusp of a normal upper molariform 
tooth, whether that element is regarded as the original primary 
cusp or otherwise. 

The objection that the terms are not universally applicable is 
scarcely worthy of consideration since they are widely appli- 
cable to the great majority of mammalian molar types, without 
in the least interfering with the use of terms descriptive of " rel- 
ative position only," which may be used in any cases where Os- 
born's terms do not apply. 



EXPLANATION OF PLATE IV. 

(All figures except fig. 9, three times natural size.) 

Fig. I. Potamogale — left upper jaw (No. 124327 U. S. N. M.) ; habitat, Africa. 

Fig. 2. Solenodon — left upper jaw (No. 2230, U. S. N. M.) ; habitat, Cuba. 

Fig. 3. Cetitetes — left upper jaw (No. 63316 U. S. N. M.) ; habitat, Mada- 
gascar. 

Fig. 4. Ericulus — left upper jaw (No. 122488 U. S. N. M.) ; habitat, Mada- 
gascar. 

Fig. 5. Hemicentetes — left upper jaw (No. 63319 U. S. N. M. ) ; habitat, Africa. 

Fig. 6. Chrysochloris — left upper jaw (No. 61686 U. S. N. M.) ; habitat, Africa. 

Fig. 7. Vespertilto fuscus — lelft upper jaw (No. 62736 U. S. N. M.) ; habitat, 
Washington, D. C. 

Fig. 8. Scoto;philus kuhli — left upper jaw (No. 113463 U. S. N. M.) ; habitat, 
Philippines. 

Fig. 9. Harpiocephaliis — right upper jaw. (Outline drawing taken from a plate 
prepared in 1880 by Wilhelm Peters for a monograph of the bats. This 
monograph was never published.) 



Proc. Wash. Acad. Sci., Vol. VIII. 



Plate IV. 




l^\y 









CHEEK TEETH OF LIVING INSECTIVORES AND BATS 



EXPLANATION OF PLATE V.' 

Figs, i and la. Trico7iodo7i / bisulcus Marsh (Atlantosaurus bedsj, left upper 

molars, m^ and m^, crown and external views. Six times natural size 

(No. 2698 U. S. N. M.). 
Figs. 2., 2a and 2b. Dryolestes sp. (Atlantosaurus beds), left upper molars; 

crown, external, and posterior views. Seven times natural size (No. 

2845 U. S. N. M.). 
Fig. 3. -Dryolestes, first right upper molar, m' ; crown view. Eight times natural 

size (No. 2839 U. S. N. M.). 
Figs. 4 and 4a. Dicrocy7iodo7i sp. (Atlantosaurus beds), left upper molars. 

crown and external views. Six times natural size (No. 271s U. S. N. 

M.). 
Figs. 5, 5«, ^b and 5c. Paurodoii sp. (Atlantosaurus beds), right lower molar, 

mo, crown, external, internal and posterior views. Eight times natural 

size (No. 2733 U. S. N. M.). 
Figs. 6, 6a, (>b and 6c. f Pediomys sp. (Laramie beds), left upper molar; crown, 

external, posterior, and anterior views. Eight times natural size (No. 

5062 U. S. N. M.). 
Figs. 7, 7a and 'jb. Gen. et sp. indt. (Laramie beds), left upper molar; crown, 

external and anterior views. Eight times natural size (No. 5076 U. S. 

N. M.). 



Proc. Wash. AcAd. Sci., Vol. VIII. 



Plate V. 




TEETH OF MESOZOIC MAMMALS 



GB^SS 










